Combine 125 grams einkorn flour, butter, sourdough starter and water in a food processor, and process until the the butter is about the size of peas and the dough barely comes together.
Transfer the dough to a floured work surface and press it together to form a ball. Cover the ball of dough with. a cotton tea towel or plastic wrap, and transfer to the fridge for 30 minutes.
Remove the dough from the fridge, uncover it and press it into a disc. Roll the dough into a circle about 12 inches in diameter.
Finishing the galette
Line a baking sheet with parchment paper and heat the oven to 400 F. Transfer the dough to the prepared baking sheet.
In a medium bowl, combine 2 tablespoons all-purpose einkorn flour, sugar, and cinnamon.
Sprinkle about 1 tablespoon of the cinnamon mix over the dough. Then add the apples and brandy to the remaining cinnamon mix and stir gently until the apples are evenly coated.
Mound the apples in the center of the crust, leaving about 1 ½ inches space at the edges. Carefully fold the edges over so that it barely overlaps the apples, crimping the dough as necessary.
Finishing the galette
In a small bowl, whisk the egg yolk and water together until uniformly combined. Brush over the crust.
Transfer the galette to the oven and bake until the crust is golden-brown and crisp and the apples turn tender -- about 35 minutes. Transfer to a wire rack to cool. Serve at room temperature.